I don't know how you can investigate when the Democrats hold all the levers of power at the state and Federal levels. Someone saying he was told he already voted or that he received multiple ballots indicate possible fraud. Trying to prove it by pointing out statistical anomalies may be a tool, but most people, and even some conservatives, fail to understand. In any case, the California courts are not inclined to accept this evidence.
The Democrats were embarrassed by the recall removals of Grey Davis and Rose Bird from office. They will not allow that to happen again.
I wish I could be more optimistic.